vi PREGNANCY PARTURITION PUERPERIUM 235 



with its vessels, so that all these elements are found united on 

 the ventral side of the embryo (Fig. 97). The amnion meanwhile 

 continues to dilate and encloses the abdominal peduncle from 

 the dorsal to the ventral surface, converting it first into a conduit 

 and then into a closed canal, the interior of which form as 

 diverticulum of the coelomic cavity, included in the cord by 

 being thus hemmed in. This canal contains the peduncle of 

 the umbilical vesicle or omphalo-mesenteric canal with the vessels 

 of the same name (Fig. 99). 



Fio. 97. Human ovum of fifteen to eighteen days (according to Costa). The cavity of the chorion 

 is open to show the embryo, 0'9 mm. long, and its adjuncts, ch, chorion; v.ch, chorionic 

 villi ; a, amnion ; v.o, umbilical vesicle ; i, intestine ; p, abdominal peduncle ; o, umbilical 

 veins. 



We may therefore say that the umbilical cord is formed of 

 the old elements of the abdominal peduncle (mesenchyme, allantoid 

 canal and umbilical vessels) with the addition of the omphalo- 

 mesenteric canal with its vessels and the diverticulum of the 

 cavity of the coelom ; all these elements of the cord are covered 

 by the amniotic epithelium. 



The cavity of the amnion steadily increases and takes the 

 place of the cavity of the chorion or external coelom, which 



